1. Field of the Invention
This invention describes a method of saving power in an implantable device, such as a pacemaker, which includes RF telemetry functionality.
2. Description of Related Art
Since the allocation of a special frequency band for implantable medical devices using RF telemetry, the so-called MICS (Medical Implantable Communication Service) band, by FCC in the late 1990's, the development of devices including this functionality has really taken off. However, since the battery capacity in an implantable device is very limited, the introduction of a RF transceiver operating at 402-405 MHz becomes a real challenge. If the transceiver operates at 5 mA in the active mode, this might be acceptable since in the normal user scenario the on-time is only a fraction (<0.01%) of the total device life time. A trickier problem is the issue of waking up the RF component from the off state to start communications in a reasonable amount of time without draining the battery.
The most common method of solving this problem today is to introduce the so-called sniff mode. This means that the complete receiver RF portion of the device is turned on for a limited period of time (e.g. 10 milliseconds) during which time the device listens to see if there are any transmitters active in the vicinity wanting to make contact. By duty cycling the on (sniff) time heavily with the off time a considerable power saving can be achieved. For example having the device on for 5 ms consuming 5 mA and then off for 995 ms while consuming only leakage current of maybe 100 nA will lower the average current consumption to only about 25 μA. This is very good in most applications. However, for an implantable device consuming less than 10 μA in total this is unacceptable. Lowering the average power consumption further by decreasing the on time is difficult since a certain time is needed to start up the RF receiver and to receive a message telling the device to start transmitting a response. Increasing the off time is not preferred since the doctor who is trying to get in contact with the device expects a response within a second or two.
An example of the prior art is found in U.S. Pat. No. 4,519,401 issued May 28, 1985.